Some engines include a variable valve train module which controls valve lift through hydraulic fluid operation. The UniAir® system which is available from Schaeffler Technologies, the assignee of the present invention, is an example of such a variable valve train system and includes a housing for hydraulic components which allows for full control over the lift profile of an associated engine valve. The housing includes various spaces for components and cavities for hydraulic fluid which together control valve timing. Hydraulic fluid systems provide fully variable valve lift capabilities, which promotes engine efficiency (e.g., through precise variable valve actuation and timing depending on the situation).
During operation of the hydraulic fluid system, a high pressure chamber is periodically pressurized and drained. This allows the hydraulic fluid in the high pressure chamber to be used as a hydraulic pushrod to open the engine valve when needed or as a disconnection which produces zero or limited lift. However, the cyclical pressurizing and depressurizing of the high pressure chamber may produce pressure fluctuations which are severe enough to produce undesirable engine noise. For example, during a late engine valve opening mode of the UniAir® system, pressure oscillation occurs in the high pressure chamber due to instantaneous closing of a control valve arranged between the high pressure chamber and a middle pressure chamber while a pump piston assembly compresses hydraulic fluid in the high pressure chamber. These pressure oscillations may contribute to noise, vibration, and harshness (NVH) issues, and it would be desirable to reduce the magnitude of these pressure oscillations.